Technical Field of the Invention
This invention is directed to a demand flow Joule-Thomson cryostat having structure adjacent the expansion nozzle to control flow of the liquid and gaseous cryogen to stabilize temperature at the point of refrigeration load.
J. S. Buller, M. J. Nagy and E. W. Peterson, U.S. Pat. No. 3,640,091 shows a Joule-Thomson cryostat assembly which has a finned cryogen delivery tube spirally surrounding an internal cylinder. The terminating end of the tube is at an expansion nozzle located adjacent to the refrigeration load. The central cylinder contains a temperature sensitive device, such as a bellows having a fluid therein which changes pressure with temperature. Attached to the bellows is an expansion valve needle in operative relationship with the expansion nozzle. As the temperature at the bellows varies, in accordance with cryogen flowing away from the expansion valve, the bellows controls the amount of cryogen expanding out of the expansion nozzle. Values are chosen to stabilize the temperature at a point where the outflow from the expansion nozzle is a mixed fluid of liquid and gas. For example, when nitrogen is used as the cryogenic fluid, a suitable temperature is 77.degree. Kelvin.
When the thermal load being cooled by the cold finger is an electronic device, the stability of the function of the device is related to the stability of the temperature. In devices such as infrared detectors, changes in temperature change the sensitivity to result in electronic instability and electronic noise. It is desirable to maintain the temperature of the detector as constant as possible to eliminate this variable from the operation of the device. As is described hereafter, the temperature of the prior art devices has not been as closely controlled as possible.